Since computers came into wide-spread use in recent years, the need for higher performance, such as miniaturization and higher capacity of hard-disks, which are one type of external storage devices, has become very high. There has been much research on lower-flying magnetic heads, for which extremely high precision of the smoothness of the magnetic disk is required.
Conventionally, aluminum alloys mainly have been used as the substrate material for magnetic disks. However, aluminum alloy substrates have a low hardness, so that their surface deforms plastically when polished, even when polishing them precisely with high-precision abrasive and polishing machinery. Therefore, it was difficult to obtain smooth surfaces with high precision. Also, forming a nickel-phosphor layer with higher hardness on the substrate surface has proven insufficient to respond to the need for smooth surfaces with high precision.
In the drive for miniaturization, disk thickness close to those of thin films are required. However, it has proven difficult to respond to this need with aluminum alloy substrates of low strength.
The recent adoption of high-sensitivity magnetoresistant heads (MR heads) has led to the need for noise reduction in the magnetic disk, and a magnetic film on the substrate has been demanded. However, it is also becoming difficult to respond to this need with aluminum alloy substrates.
Therefore, new materials such as glass, ceramics and carbon have been suggested for magnetic-disk substrates. Of these materials, glass substrates have been studied in detail and have reached practical use already. Glass substrates have high strength, good heat resistance, and high surface hardness, and can satisfy the need for very precise smoothness by being precision-polished with high accuracy.
In magnetic disks with high surface smoothness, the flying height of the magnetic head can be reduced, but on the other hand the so-called "head-stick" problem occurs. This means, in a hard-disk drive employing the contact-start-stop (CSS) method, the magnetic head and the magnetic disk come in sliding contact when the device is started or stopped. Because the rotation is started or stopped in this situation, a friction force arises between the magnetic head and the magnetic disk, and the surfaces of the magnetic head and the magnetic disk are abraded, which becomes a reason for the deterioration of the magnetic conversion properties.
Especially when water adheres to the magnetic disk, water enters the space between the magnetic head and the magnetic disk and leads to agglutination. When the hard-disk is started in this situation, a large resistance occurs between the magnetic head and the magnetic disk, which sometimes leads to damage or destruction of the magnetic disk, so the head-stick phenomenon is a serious problem.
To achieve both a lower-flying magnetic head and prevent adhesion, there have been attempts to reduce the surface roughness as much as possible. However, this method reached a limit as to how low the magnetic head can float. As another method to achieve both goals, a so-called zone-texture technique has been suggested, with a CSS zone, where the magnetic head takes off and lands on the disk surface (for example, a doughnut-shaped region near the inner circumference of the disk), whose surface is kept rough enough so that magnetic head/disk adhesion and damage can be prevented, and a data zone where the surface roughness is smaller than in the CSS zone, so that a lower-flying head can be realized.
Conventionally, to obtain magnetic-disk glass substrates with a smooth surface, glass substrates were cut into predetermined sizes and precision-polished one by one. Publications of Unexamined Japanese Patent Application (Tokkai) No. Sho 63-152022 and Sho 63-167422 disclose a method for partially changing the degree of the polishing step and a method for partially etching, as methods for partially forming irregularities in the surface of the magnetic disk substrate. Moreover, a method for surface processing using a laser has been disclosed recently in Publication of Unexamined Japanese Patent Application (Tokkai) No. Hei 7-182655. This method is beginning to be researched widely as a method with good controllability.
Press-molding has been researched widely in the field of manufacturing optical glass elements and there have been efforts to put it into practical use as a manufacturing method with high product quality and high productivity.
A special die is necessary for press-molding, that is, a die that does not deteriorate when molding glass repeatedly under high temperature and high pressure, and several types of molds have been researched.
Super-hard alloys (tungsten carbide), cermet, zirconia, silicon carbide and ceramics are used for the press-molding die base material. Molds have been developed that are coated with a protective film with good releasability, oxidation resistance, and reaction resistance, in order to protect the die base material and prevent sticking of the glass at die release.
For example, Publication of Unexamined Japanese Patent Application (Tokkai) No. Hei 2-137914 suggests a casting mold, comprising a noble metal alloy thin film on a super-hard alloy surface, where a fine pattern is formed on the alloy surface.
However, conventional manufacturing methods for magnetic disk substrates as described above lead to the following problems:
(1) When precision-polishing the glass substrates one by one, there is the problem that high precision is required in the polishing step, and that the number of steps is large. PA1 (2) When partially changing the degree of the polishing step or partially etching, there is a limit as to how precise the surface properties can be controlled while maintaining good productivity. PA1 (3) Using a laser to process the surface requires costly machinery, so that the problem of higher manufacturing costs arises. PA1 (4) In the press-molding method, there is the problem that the processability of the super-hard alloy (tungsten carbide) or the cermet used for the die base material is low, and that it is difficult to obtain a sufficient smoothness (of nm order) for the magnetic-disk substrate mold.
It is a purpose of the present invention to provide a press-molding die and a magnetic-disk glass substrate to solve these problems. It is a further purpose of the present invention to provide a press-molding die that can manufacture magnetic disks that are suitable for the zone-texture technique in large quantities and at a low price by forming smooth surface areas and rough surface areas on the surface of the press-molding die. It is a further purpose of the present invention to provide a manufacturing method for this press-molding die. It is a further purpose of the present invention to provide a magnetic-disk glass substrate, formed using this press-molding die, that has improved CSS properties and allows a lower flying height.